Electrical windings

ABSTRACT

An electrical winding structure having first and second tapped winding portions, and a switching device which selects and interconnects taps on the two winding portions. A capacitor is electrically connected between the two winding portions at the coil break, i.e., between the ends of the two winding sections which are &#39;&#39;&#39;&#39;floating&#39;&#39;&#39;&#39; when the switching device is not in the maximum turn position, to reduce the magnitude of surge stresses across the coil break.

United States Patent App]. No: 67,179

US. Cl ..3l7/l5, 3 l7/DIG. 6, 323/435 R,

Yannucci' Feb. 22, 1972 [54] ELECTRICAL WINDINGS Primary Examiner-Thomas .l. Kozma l 72 Inventor: Dean A. Yannucci, Niles, Ohio Q32? A T stratum F E Bmwde Dmald [73] Assignee: Westinghouse Electric Coi-poration, Pittsburgh, Pa. 221 Filed: Aug.26, 1970 [571 ABSTRACT [21 1 An electrical winding structure having first and second tapped winding portions, and a switching device which selects and in- [52] tel-connects taps on the two winding portionsv A capacitor is 336/150 electrically connected between the two winding portions at [511' Int Cl 02h 7/04 the coil break, i.e., between the ends of the two winding sec- [58] Fieid 146 147 tions which are floating" when the switching device IS not m 336/150 5 4 the maximum tum position, to reduce the magnitude of surge DIG 6 stresses acgossthe coil break. [56] References Cited UNITED STATES PATENTS 1,658,664 2/1928 Brand ..323/43.5 vsclmjmanmwing 1,873,824 8/1932 Cole et al.... ...336/l 3,452,311 6/1969 Beck et al... .....336/ 3,332,050 7/1967 Vmgo "3231435 B A B A B A B A B A I 76 78 E t a u e5 ,7? E I Q 68 I I 80 82 E: I I g 1 i \1 87 I '79 U $1 a I 34 {E j w r/ 3p C i I 74 -I22 aegi I t 36 TIBOFOJRDB 1:2 I

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sum 2 OF 2 OF TOTAL TURNS TAPPED OUT Field of the Invention The invention relates in general to winding structures for electrical inductive apparatus, such as power transformers, and more specifically to tapped winding structures for such apparatus.

Description of the Prior Art In the application of no-load taps to electrical transformers, the tap section, or sections, of each phase winding conventionally has an electrical discontinuity, termed the coil or tap break, thus dividing the tap section into first and second sections, with taps disposed on each section. Suitable no-load switching or tap changer means is disposed to bridge the coil break, selectively interconnecting taps on the two sections, to select the desired turn ratio between the high and low voltage windings of the transformer.

Extreme care must be taken in design of the tap section, whether the transformer is of the pancake coil or of the cylindrical winding type. For example, in power transformers ofthe core-form type, having axially spaced pancake type coils in the high voltage winding, the tap section should be located at the electrical center of each phase, i.e., the center-line which divides the ampere turns into two equal portions, in order to produce minimal axial unbalance and thus reduce the varia tion in impedance over the tap range, and also reduce the magnitude of short circuit forces caused by the unbalance. If the tap range exceeds about percent of the total turns of the phase, it may be necessary to utilize two tap sections, spaced apart in the phase, to reduce the size of the gap in the ampere turns produced in the winding when all of the turns of the tap section are tapped out, which gap allows the leakage flux to fringe outwardly and cause stray losses and tank wall heating. The greater the percentage of the winding tapped out by a tap section, the greater the steady state voltage stress between the two portions of the tap section at the coil break. If the full wave point-to-point strength of the tap changer is exceeded when the turns are tapped out, the tap section must necessarily be divided into two spaced tap sections.

In cylindrical coils i.e., those having a plurality of radial layers of turns instead of axially spaced pancake coils, the taps must be carefully located. For example, the coil diameter should not be reduced as the winding is tapped out, as any radial or axial unbalance will produce an impedance change.

After carefully selecting the location of the tap section, or sections, and the taps therein, determining if more than one tap section should be utilized, and the insulating spacing required between the portions of the tap section at the coil break, voltage breakdown may still occur at the coil break during a surge condition, especially when all ofthe turns of the tap section are tapped out. Thus, the coil break is heavily insulated to withstand surge stresses, which require additional space between the two portions of the tap section, requiring a larger magnetic core than would otherwise be necessary, which deleteriously affects the weight and cost of the apparatus.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved electrical winding for inductive apparatus which includes at least one tap section having first and second portions electrically separated by a coil break, and tap changer means disposed to interconnect taps on the two portions. The ends of the two portions of the tap section, at the coil break, are interconnected via a discrete capacitor. It has been found that by capacitively coupling the ends of the two portions ofa tap section, the magnitude of the low frequency oscillatory voltage produced by an impulse voltage wave is remarkably reduced, and the frequency of oscillation is substantially lowered, to a value which is not a multiple of the fundamental frequency. The insulating clearances between the two portions of the lap section, at the coil break, may therefore be substantially reduced, effecting economies in the size, weight and cost of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description of exemplary embodiments thereof, taken with the accompanying drawings, in which:

FIG. 1 is a partial sectional elevation of a transformer having a tapped winding of the interleaved turn, high series capacitance pancake coil type, constructed according to the teachings of the invention;

FIG. 2 is a partial sectional elevation of a transformer having a tapped winding of the continuous pancake type constructed according to the teachings of the invention; 3

FIG. 3 is a graph which illustrates how the surge stress increases across a tap break as the turns are tapped out.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown an elevational view of a transformer 10 of the core form type, constructed according to the teachings of the invention, which may be single-phase or poly-phase, as desired. Since the invention may be adequately illustrated by a portion ofa single-phase winding assembly, only a single phase is shown in the elevational view of transformer 10 in FIG. 1.

Transformer 10 includes a magnetic core 12 which may be of conventional construction, having a winding leg 14 about which high and low voltage phase winding assemblies 16 and 18 are concentrically disposed. Low voltage winding 18 may also be of conventional construction, having a plurality of electrically insulated conductor turns 20, which are insulated from magnetic core 12 and high voltage winding assembly 16 by insulating means 22.

High voltage phase winding assembly 16 comprises a plurality of pancake or disc-type coils which are spaced axially apart in a stacked arrangement about the longitudinal centerline 24 of magnetic core leg 14. Winding 16 has terminals 26 and 28 at the ends thereof, which are connected to pancake coils 30 and 32, respectively, and a tap section 34 includes a plurality of pancake coils, such as pancake coils 38, 40, 42, 44, 46, 48, 50 and 52. The tap section 34 is disposed intermediate the ends of winding 16, with a plurality of pancake coils, indicated generally be pancake coils 30 and 36 and a line 56 between them, being disposed between one end of the tap section and one end of the winding 16, and a plurality of pancake coils, indicated generally by pancake coils 54 and 32 and a line 58 between them, disposed between the other end of the tap section 34 and the other end of the winding 16.

In this embodiment, the plurality of pancake coils, such as pancake coil 30, are each constructed and connected to increase their series capacitance, and thus the series capacitance of the winding structure 16. The series capacitance of the pancake coils is increased by disposing turns from an electrically distant portion of the coil, or winding, between electrically connected turns. This method of increasing series capacitance, called interleaving, may be accomplished by any one of several different arrangements. U.S.

Pat. Nos. 3,090,022, 3,260,978, 3,278,879, and 3,299,385 all disclose suitable interleaving arrangements which may be used. For purposes of example, the interleaving arrangement disclosed in US. Pat. No. 3,090,022, called single interleaving, is illustrated in FIG. I. In general, in the single interleaving arrangement, each pancake coil is wound with first and second electrical conductors to provide first and second interleaved coil sections each having a plurality of conductor turns which are radially interleaved with one another. The interleaving of each pancake coil is performed by connecting the inner end of one coil section to the outer end of the other coil section. The interleaved pancake coils are connected in series with one another, to complete the winding structure. The pancake coils may be connected start-start, finish-finish; or, finish-start. The start of a coil is the inner end of one of its coil sections, and the finish" is the outer end of one of its coil sections, regardless of where the winding circuit first enters the pancake coil.

More specifically, FIG. 1 illustrates the pancake coils of winding 16 singly interleaved and start-start, finish-finish connected. The first pancake coil 30 includes first and second coil sections with the circuit first entering the outer end of the first coil section from the line terminal 26 via conductor 60, and spiraling inwardly via every other turn, appearing at turns A1, A2, A3 and A4. These conductor turns are given the letter A to denote the first excursion of the circuit through the coil, and a number to indicate its electrical position in the winding. At the inner end of the first coil section, the circuit proceeds via interleaving connection 62 to the outer end of the second coil section, with this end being referenced B4 to indicate the start of the second excursion of the circuit through the pancake coil 30, and to indicate that it is electrically at substantially the same potential as the end of turn A4. The circuit again spirals inwardly through pancake coil 30, appearing at every other turn B5, B6, B7 and B8. This completes pancake coil 30, and the end of turn B8 is connected to the inner end of one of the coil sections of the next adjacent pancake coil, such as the first coil section, via start-start connection 64. The circuit spirals outwardly through the next pancake coil, appearing at every other turn and the outer end of the coil section initially traversed is connected to the inner end of the other coil section, via an interleaving connection. The circuit again spirals outwardly through this pancake coil, appearing at every other turn. This completes the second pancake coil. The remaining pairs of pancake coils are all constructed and interconnected in a manner similar to the first pair. Since the circuit spirals inwardly in one pancake coil of a pair and outwardly in the other pancake coil of the pair, the turns in the two pancake coils of each pair should spiral in opposite circumferential directions, in order to each provide a magnetomotive force in the magnetic circuit which is in the same direction.

In order to vary the effective-number of turns in winding 16, and thus vary the turn ratio of transformer 10, as required by the specific location of the transformer in an electrical power system, or in response to load conditions on the transformer, winding 16 includes a tapv section 34. Winding 16 is illustrated with a single tap section, by way of example, but it will be understood that it may have any desired number.

Tap section 34 is divided into first and second portions 68 and 70, respectively, by an electrical discontinuity or break in the winding, with the tap or coil break being .bridgedvia switching or tap changer means 66. The first portion 68 of the tap section 34 includes pancake coils 38, 40, 42, and 44, and the second portion 70 of tap section 34 includes pancake coils 46, 48, and 52. lnstead of interconnecting the adjacent pancake coils 44 and 46 directly together via a finish-finish connection, the outer ends of these two coils are connected to the switching means 66 via leads 72 and 74, respectively, which form two tap leads of the tap section. Tap section 34 is further tapped at the finish-finish connection 76 between pancake coils 36 and 38, with this tap being connected to switching means 66 via conductor 78, at the start-start connection 77 between pancake coils 38 and 40, via conductor 85, at the finish-finish connection 80 between pancake coils 40 and 42 via conductor 82, at the start-start connection 79 between pancake coils 42 and 44 via the conductor 87, at the start-start connection 81 between pancake coils 46 and 48 via conductor 89, at the finish-finish connection 84 between pancake coils 48 and 50 via conductor 86, at the start-start connection 83 between pancake coils 50 and 52 via conductor 91, and at the finish-finish connection 88 between pancake coils 52 and 54 via conductor 90. The specific number of pancake coils in the tap section 34 will depend upon the application, and the location of the taps depend upon the type of pancake coils and their interleaving arrangement.

Lumped capacitance Switching or tap changing means 66 may be of any desired construction, and is illustrated only functionally in FIG. 1. Switching means 66 is illustrated as having five operating positions, with a shorting bar 92 disposed to selectively short the terminals of a desired position. The first position has terminals 94 and 96 which interconnect tap leads 72 and 74, and is the maximum turn position of the switch, shorting no turns of the tap section. The next position includes terminals 98 and 100 which interconnect tap leads 87 and 89, the next position includes terminals 102 and 104 which interconnect tap leads 82 and 86, the next position includes terminals 106 and 108 which interconnect tap leads 85 and 91, and the final position includes terminals 110 and 112 which interconnect tap leads 78 and 90, and is the minimum turn position which shorts out all of the turns of the tap section 34.

When the tap changing means 66 is operated to move from the maximum turn to the minimum turn position, the impulse voltage stress appearing across the coil break, between pancake coils 44 and 46 increases, with the maximum stress being in the minimum turn position illustrated in FIG. 1.

FIG. 3 is a graph which illustrates how the voltage stress increases across the tap break of a winding constructed of singly interleaved pancake coils, such as illustrated in FIG. 1, as the turns are tapped out, with the percent of the applied voltage wave which appears across the coil break, compared with the,

magnitude of the applied voltage wave, being measured on the ordinate, and the percentage of the total turns tapped out is plotted on the abscissa. The winding was surge tested in air with a grounded low voltage winding inside it, using a l.5 40 full wave impulse voltage. Surge testing interleaved turn, high series capacitance windings in air has been found to accurately predict the results in oil, since the high series capacitance is developed primarily between the turns of a pancake, and not between the pancake coils. Since the dielectric constant of the insulation between conductor turns of a pancake coil is substantially the same whether the coil is in air or oil, the results provided by surge testing are substantially the same. It will be noted from curve 120 in FIG. 3, which was developed by testing the singly interleaved winding and connecting the test points, that the percentage of the impulse wave which appears across the coil break is almost directly proportional to the percentage of the number of turns tapped out of the winding. With a conventional 10 percent tapping, the percentage of the applied wave appearing across the coil break is about 8 or 9 percent, which requires a substantial insulating clearance at the tap break, for example, about 0.75 inches in a singly interleaved high series capacitance winding for a transformer rated l0,000 kva. at kv.

The surge or impulse stresses may be markedly reduced, according to the teachings of the invention, by coupling the two winding portions 68 and 70 at the tap break with a capacitor 122, which interconnects tap leads 72 and 74. Capacitor 122 has been found to substantially reduce the impulse stresses appearing at the coil break, by capacitively coupling the ends of the coil sections and preventing them from oscillating out of phase with respect to one another. The capacitor tends to equalize the voltage at these two points of the tap break, which points are floating when the tap changer is not directly interconnecting them, and it also reduces the frequency of the oscillations of the winding when the winding is subjected to a surge voltage.

The following table illustrates the affect the amount of lumped capacitance has on the percent of the applied impulse wave which appears across the tap break, and also its affect on the frequency of oscillation.

Percent of applied Wave across coil Frequency of (farads) Break oscillation (Hz,)

As shown in the table, 0.5 microfarads of capacitance connected across the tap break reduces the impulse stress to 2 percent of the applied wave, a reduction of 79 percent from the 9.4 percent experienced without the discrete capacitor, and the frequency of oscillation is reduced from 1 1,750 Hz. to 667 Hz. The 79 percent reduction enables the tap or coil break clearance to be reduced from about 0.75 inches to about 0.185 inches in the high-series capacitance winding hereinbefore referred to, which has the advantage of reducing the stack dimension of the coils, the length of the winding leg required, and thus the amount of weight of iron required in the magnetic core.

For a percent tap out and a rated voltage of 115 kv., using a capacitor rated 0.5 microfarads across the coil break, the circulating current caused by the capacitor resulted in a negligible loss of about 62 watts. I

FIG. 2 is a partial elevational view of a-single phase of a transformer 130, which illustrates the application of the invention to a transformer of the core-form type having continuous windings, instead of windings of the interleaved turn, high se-- ries capacitance type. Transformer 130 includes a magnetic core-winding assembly 132 having high and low voltage windings 134 and 136 respectively, concentrically disposed about a winding leg 138 of the magnetic core, with the high and low voltage windings 134 and 136 being symmetrical about the longitudinal centerline 140 of the winding leg 138. The high voltage winding 134 is connected between terminals 142 and 144, with a pancake coil 146 at the first end of the winding being connected to terminal 142, and a pancake coil 148 at the other end of the winding being connected to terminal 144. The high voltage winding 134 has a tap section 150 disposed between its ends, and connected to pancake coils 152 and 162 adjacent the ends of the winding, with tap section 150 being illustrated as having pancake coils 154, 156, 158 and 160. Each of the pancake coils of the winding are of the continuous type, wherein the turns are mechanically and electrically adjacent one another, as illustrated relative to pancake coil 146 in FIG. 2. The pancake coils are connected in series across the winding with start-start, finish-finish connections, with pancake coils 152 and 154 being interconnected with finish-finish connection 180, pancake coils 154 and 156 being interconnected with start-start connection 184, pancake coils 158 and 160 being interconnected with start-start connection 188, and pancake coils 160 and 162 being interconnected with finish-finish connection 192.

A tap changer 164 having terminals 166, 168, 170, 172, 174 and 176 is disposed and connected to change taps on the tap section 150 in response to the position of its shorting bar 178. The outer end of pancake coil 156 is connected to terminal 166 ofa tap changer means 164 via lead 187, the outer end of pancake coil 158 is connected to terminal 168 via conductor 189, the start-start connection 184 is connected to terminal 170 via electrical conductor 186, the start-start connection 188 is connected to terminal 172 via conductor 190, finishfinish connection 180 is connected to terminal 174 via conductor 182, and the finish-finish connection 192 is connected to terminal 176 via conductor 194.

A capacitor 199, according to the teachings of the invention, is connected directly across the tap break between conductors 187 and 189. The first position of the tap changer means 164 shorts ten-ninals 1 66 and 168, and is the maximum turn position of the switch, as it connects all of the turns of the tap section into the high voltage winding 134. In the second positionof tap changer switch 164, terminals 170 and 172 are shorted, which removes the turns of pancake coils 156 and 158 from the winding, and leaves them electrically floating. Capacitor 199, as hereinbefore described relative to the embodiment of the invention shown in FIG. 1, prevents the pancake coils 156 and 158 from oscillating widely out of phase, and prevents a high voltage buildup between the ends of these pancake coils. In the third position of the tap changer switch 164, terminals 174 and 176 are shorted, as illustrated in FIG. 2, which effectively removes ancake coils 154, 156, 158 and from the circuit, but whiclestill leaves these coils electrically floating. Capacitor 199 again performs its function of reducing the magnitude of the oscillatory voltages produced between the tap break when the winding is subjected to a surge potential.

In summary, there has been disclosed a new and improved electrical winding structure for electrical inductive apparatus, such as power transfonners of the type which has first and second tapped winding portions interconnected with a switching device which changes the effective number of turns in the winding. The capacitor is connected across the tap break between the tapped winding portions, which substantially reduces the magnitude of surge stresses appearing across the tap break when the tap changer is not in its maximum turn position. The use of a capacitor connected across the tap break enables the insulating clearance at this point to be reduced, which reduces the overall length of the winding stack, and thus the length of the magnetic core leg required. Therefore, for the cost of a small discrete capacitor, substantial savings may be made in the size, weight and cost of the apparatus.

I claim as my invention:

1. A winding assembly for electrical inductive apparatus, comprising:

first and second winding portions each having first and second ends and each having a plurality of conductor turns connected between their ends,

a plurality of taps connected to predetermined conductor turns of said first and second winding portions,

switching means electrically interconnecting said first and second winding portions, said switching means being operable to select a tap on each of said first and second winding portions and provide at least one series path between the first and second ends of said first and second winding portions, respectively, having a preselectable number of turns therein,

and a capacitor electrically connected between said first and second winding portions, said capacitor being directly connected to the second and first ends of said first and second winding portions, respectively, to reduce the magnitude of surge voltage oscillations between their adjacent ends.

2. The winding of claim 1 wherein each winding portion includes at least one pancake coil of the continuous type.

3. The winding of claim 1 wherein each winding portion includes at least one pancake coil of the interleaved turn, highseries capacitance type. 

1. A winding assembly for electrical inductive apparatus, comprising: first and second winding portions each having first and second ends and each having a plurality of conductor turns connected between their ends, a plurality of taps connected to predetermined conductor turns of said first and second winding portions, switching means electrically interconnecting said first and second winding portions, said switching means being operaBle to select a tap on each of said first and second winding portions and provide at least one series path between the first and second ends of said first and second winding portions, respectively, having a preselectable number of turns therein, and a capacitor electrically connected between said first and second winding portions, said capacitor being directly connected to the second and first ends of said first and second winding portions, respectively, to reduce the magnitude of surge voltage oscillations between their adjacent ends.
 2. The winding of claim 1 wherein each winding portion includes at least one pancake coil of the continuous type.
 3. The winding of claim 1 wherein each winding portion includes at least one pancake coil of the interleaved turn, high-series capacitance type. 